Old English tiwesdæg, from Tiwes, genitive of Tiw “Tiu,” from Proto-Germanic *Tiwaz “god of the sky,” the original supreme deity of ancient Germanic mythology, differentiated specifically as Tiu, ancient Germanic god of war, from PIE *deiwos “god,” from root *dyeu- “to shine,” in derivatives “sky, heaven, god.” Cognate with Old Frisian tiesdei, Old Norse tysdagr, Swedish tisdag, Old High German ziestag.